A common type of electrophotographic copier machine calls for exposing an original document to produce an image on a photoreceptive medium. The image is then developed by placing toning material on the photoreceptive medium, and if the photoreceptive medium is not also the copy paper, the developed image must be transferred to copy paper.
In order to produce a finished copy, the toning material is bonded to the copy paper in a permanent manner. One satisfactory device to accomplish bonding is to send the paper through the nip of a pair of fusing rollers whereat pressure and usually heat are applied to fuse the image onto the copy paper.
A well-known problem of the roll fuser has been the tendency of copy paper to wrinkle under high humidity conditions. To remedy that problem, it has been suggested in the prior art that a taper be provided on one of the rolls so as to produce a concave roller. The object of the taper is to produce a higher peripheral speed near the edge of the roller so that as copy paper passes through the nip of the mating rollers the peripheral speed at the edge is greater than it is in the middle. As a result, the paper tends to stretch and does not wrinkle even under high humidity conditions.
Unfortunately, while a tapered roller alleviates the wrinkling problem, it makes worse the smearing problem which is encountered under very dry conditions. At dry conditions copy paper tends to crinkle or have small waves in it so that as the copy paper enters the fuser, premature contact with the fusing roll creates a smearing of the image. Both the wrinkling problem and the smearing problem are more severe for lightweight papers.
It is the general object of this invention to provide a roll fuser through which, by proper adjustment, the problem of wrinkling of copy paper can be eliminated at high humidity conditions and the problem of smearing can be eliminated at low humidity conditions.